digital_electricity: bcd counter tutorial

Added level 22 (final level, as of now) aims at giving a basic idea of
how the bcd counter works.

The checking procedure is a bit different from others, which is done by:

check if all the terminals for the bcd counter is connected
if false, return wrong answer
if true and  digital(valueOf(bcdCounter)) ==
digital(valueOf(digitalLights)) then correct, else wrong

Signed-off-by: Rudra Nil Basu <rudra.nil.basu.1996@gmail.com>